Introduction On October 1, 1997, the National Institute for Occupational Safety and Health (NIOSH) received a request for a Health Hazard Evaluation (HHE) at a pulp and paper plant in Minnesota. The request concerned worker exposures during the construction of a new boiler, primarily, to emissions generated from three adjacent operating boilers. The reported health effects were headache and respiratory effects, such as asthma. The construction on the new boiler began in 1996 and the number of construction workers at the site varied between 400 and 450. In response to the request, NIOSH investigators conducted an environmental and medical evaluation of the new boiler at the construction site on November 19th 21, 1997. Given the specific concerns of the workers and the possibility for stack emissions (from adjacent operating boilers) to migrate through the construction area, the sampling efforts during the NIOSH evaluation were focused on worker exposures incurred from the nearby operating boilers.